With two days to go before the all-important or (depending on who's talking) not-very-important Ames Straw Poll, GOP presidential hopefuls face off in Iowa tonight for a two-hour debate starting at 9 p.m. Here are some points to keep in mind:

The debate marks a new, more bare-knuckle phase in the presidential race, says Politico: "Forget about the pledges of civility and the widespread reluctance to engage in explicit personal attacks. The polite phase of the 2012 campaign is about to come to an end. When eight Republicans face off in the first Iowa debate Thursday night, almost all of them will be feeling pressure to get noticed – and get tougher... 'This is that window where the candidates can rise to the occasion or crumble. We’ll look back on this in about five months and the field won’t look anything like it does now because some people will have risen to this moment,' said Republican strategist Alex Castellanos, who is unaligned in the 2012 race. 'This is the beginning of the real campaign here.'” [Politico]

The results of the debate and straw poll can shape the nature of the race going forward: "Behind the festivities orchestrated by the Republican presidential candidates for the nearby Ames Straw Poll lie do-or-die stakes. Most of the GOP contenders have been furiously circling the state in the lead-up to perhaps the most important week of the campaign so far... A single bad performance in an actual caucus or a primary can be less devastating to a campaign than underperforming at the Iowa Straw Poll,’’ said Gentry Collins, an Iowan and former political director at the Republican National Committee. Just ask Lamar Alexander, Sam Brownback, Elizabeth Dole, Dan Quayle, and Tommy Thompson, all of whom bowed out of the race after weak showings in this quirky, quadrennial mock election." [National Journal]

Candidates will need to show they have a clue when it comes to the economy: "Amid new anxiety about the economy and a plunging stock market, the Republican presidential candidates in Thursday's debate will be united in their criticism of President Obama... In the midst of the nation's economic turmoil, there is little room for error. One needs only to look back to the last presidential contest, when GOP nominee John McCain argued during the 2008 market meltdown that the 'fundamentals of the economy' were strong, to see the potential for a damaging mistake. 'There are many more things that can go wrong in a debate like this than can go right,' said strategist Steve Schmidt, who guided McCain's 2008 effort. The candidates' 'challenge is to begin connecting with the Republican primary voting audience and to convey that they are the person best able to take the fight to President Obama.'" [LA Times]

One big question, as TPM's Benjy Sarlin puts it, is whether the other candidates will be able to throw Mitt Romney off his game: "His opponents, starting with Tim Pawlenty at the last debate, have failed to land a decent punch and Romney's been left free to run his campaign as if it were a general election contest with a polished set of attacks on President Obama's jobs record. Advertising money is tight for the rest of the field, so if they want to knock Romney down a peg the debates may be their best chance to make an impact. Will any of them bite?" [TPM]

Look to see if Michele Bachmann can pull off a repeat of her surprisingly assured performance in June's New Hampshire debate, writes the Times' Michael Shear: "Mrs. Bachmann’s political prowess comes in part from her willingness to take extreme positions that are characterized by the establishment as “wacky” but which fire up the conservative base. Coming into the race, her challenge was to steer a course that satisfied both of those constituencies. In her first debate appearance, she did that well, impressing Republican insiders and donors while not disappointing her core supporters." [NYT]